when you eventually upgrade to 8 speed, don't.
Just go to 9.
You need new shifters for both, or stay with friction if that's what you have.
Cost of the cassette isn't much different.
9 has way more combinations to choose from.
You need a 9 speed chain, but you'll probably be getting a new chain anyway, so that'll be just a few bucks more than an 8 speed.